Over the past twenty-five years, advanced polymeric matrix composites have become established as high-performance structural materials, but composite oxidation limits service lifetimes at elevated temperature. Protective coatings that provide improved thermo-mechanical performance at high temperatures are needed to effectively utilize polymeric matrix composites in future aerospace applications. Protection of machined surfaces, especially machined edges having exposed fibers, from rapid oxidation, is especially needed.
One widely used polymeric matrix material is an addition-type polyimide known as PMR-15. Due to PMR-15's relatively high service temperature, processing ease and good thermal stability, it has become the polyimide "standard" for the aerospace industry. However, due to oxidation, a carbon fiber/PMR-15 composite will lose about five to seven percent in compression strength for every one percent in weight loss.
A coating of a fluorinated polyimide known as L30N has been shown to protect carbon fiber/PMR-15 composites from erosion at elevated temperature. The L30N resin is currently commercially available from TRW, Inc. of Redondo Beach, Calif. as PFPI.
Thin ceramic coatings have also been investigated as oxygen barriers to protect PMR-15 substrates at elevated temperatures. In most cases, these coatings did protect the composite surface from oxidative degradation over several hundred hours of constant thermal conditioning up to 380.degree. C. (716.degree. F.). However, such ceramic coatings have an extreme mismatch in the coefficient of thermal expansion (CTE) between the ceramic protective film and the polymeric composite. The internal or compressive stresses that are formed in the ceramic coating during cooling in the CVD reactor are significant enough to cause the protective layer to crack or delaminate from the composite substrate. This CTE problem is very significant when a composite component undergoes thermal cycling during service, commonly resulting in spalling of the coating. In order to reduce the thermal stresses at the composite/coating interface, compliant layers consisting of different fluorinated high-temperature polyimides have been used.
According to the invention, there is provided a polyamic acid polyimide precursor solution suitable for use as a coating to protect polyimide substrates against thermally induced oxidative degradation. Coatings according to the invention may be sprayed or brushed on to the substrate. Following application of the polyamic acid solution, the coated substrate is heated to convert the coating to a polyimide.
The coating solution may be prepared by by reacting an aromatic dianhydride with an aromatic diamine in a non-reactive solvent and heating the solution to a temperature between 50.degree. C. and 150.degree. C., more preferably from 80.degree. C. to 145.degree. C. In certain emodiments, the reactive coating solution is preferably heated until a viscosity drop occurs. The heating may be carried out at at elevated temperature above 50.degree. C. in an inert atmosphere for a time sufficient to obtain a polyamic acid having a Brookfield viscosity of from about 500 to about 5000 cP, more preferably about 600 to about 2000 cP and a solids content of from about 5 to about 35 weight percent, more preferably from about 10 to about 30 weight percent. Suitable material has been obtained by heating the reactive mixture under nitrogen near but not at reflux for about one hour in in n-methyl pyrrolidinone solvent (NMP)
Although meta-phenylenediamine or para-phenylenediamine can be used individually, a mixture of meta-phenylenediamine and para-phenylenediamine preferably is used in combination with 3,3'4,4'-biphenyldianhydride. The meta-phenylenediamine isomer is believed to provide a "kink" in the polymer chain backbone as contrasted with a straight polymer chain backbone which is expected if only para-phenylenediamine is used. This was done in an attempt to create a less brittle and more ductile coating than would result if only para-phenylenediamine was used. The ratio of meta-PDA to para-PDA is preferably from about 90:10 to about 15:85, and more preferably, from about 75:25 to about 50:50.
It is believed that 2,3,3',4'-biphenyldianhydride can be used as a total or partial substitute for 3,3'4,4'-biphenyldianhydride.
It is believed that a suitable coating can be made by reacting 3,3',4,4'-biphenyldianhydride (BPDA) with a 0.8 to 1.2:1, preferably 0.9 to 1.1:1 stoichiometric ratio of meta-phenylenediamine and para-phenylenediamine in NMP. Stoichiometric excess of PDA will result in excess amine linkages. Stoichiometric excess of BPDA is believed preferable to provide excess carboxyl groups which are expected to enhance adhesion of the coating to a polyimide substrate.
The inclusion of one or more diamines or dianhydrides other than those disclosed, e.g. aliphatic diamines or aliphatic dianhydrides or other aromatic diamines and dianhydrides, as reactants in the process may detract from one or more desirable properties of the polymeric products. However, the inclusion of such materials, to the extent that they do not detract substantially from the desirable results obtained with the stated reactants, is contemplated in the formation of the corresponding copolymers.
The reaction to form the polyamic acid is preferably carried out in a polar solvent. The most preferred solvent is n-methyl pyrrolidinone solvent (NMP). However, it is believed that other solvents such as alcohols, ketones, ethers, cyclic amides and dialkylsulfoxides may be employed as well as mixtures of solvents.